Tires: z rated tires, cadillac cts, utqg ratings

Question
I bought a 2004 cadillac cts off show room floor with z rated tires.  I have purchased one more set since then.  The size is 245-45-18.  I love the ride that I get, but am wondering if a H rated tire would give me the same ride, but more tread miles.  I am not an aggressive driver, in fact, only have 35,000 miles on this car, but have been through 2 sets, and  am in need of new tires now.

Answer
Sarah,

The problem isn't necessarily the speed rating, because some H rated tires are also designed for high grip.  But as a general rule, lower speed rated tires generally wear better, but at the sacrifice of grip.  You have to pay attention to the UTQG ratings for treadwear and traction.